The present invention relates to a scanning electron microscope and an image forming method therewith.
In the scanning electron microscope, the user can observe an enlarged image at an optional location on the sample on the monitor. Since the enlarged image is of a part of the sample, it requires a lot of time to select the target field of view. When the sample is small, the entire sample can be observed at one time by setting the magnification ratio to the minimum magnification factor. However, to search for the field of view, the field of view is moved by stage movement through observation at the minimum magnification factor.
As a means for decreasing the apparent minimum magnification factor, there is a method for displaying an image at a very low magnification factor by stage movement and connection on the frame memory as described in Japanese Patent Publication No.4-74824 (1992). The method comprises dividing the surface of a measuring sample into a plurality of sections, scanning the surfaces of the sections with an electron beam, recording the image data for each section in each frame memory for one picture, reducing each of the plural images recorded in the frame memory, connecting the reduced images with each other to form and display a picture of very low magnification and wide view image.
Since very low magnification and wide view images beyond the electron beam scanning width can be displayed, such a prior art is effective in confirming the position of the target section on the screen. However, since image data of divided sections is recorded in the frame memory and the images are reduced and connected, to obtain very low magnification and wide view images, the frame scanning time for the number of divisions and the time for image reduction and connection are required in addition to the stage moving time, accordingly a very long period of time is necessary. For example, in the case where the surface of a sample is divided into 16 sections and the frame memories of 16 in number are used, the time necessary to form very low magnification factor images can be expressed by the following formula.
[Forming time of very low magnification and wide view image]=[15 times stage moving time]+[16 times frame scanning time]+[16 times image reducing time]+[connecting time].
A memory for frames for the number of divisions is necessary in addition to the very low magnification factor image frame memory. In the example described above, the necessary memory capacity is 17 frames including very low magnification and wide view images.